1. Field of the Invention
This invention relates to a method of enhanced oil recovery utilizing a foamed carbon dioxide sweep fluid. More particularly the invention relates to such a method utilizing a foam having improved stability.
2. Description of the Prior Art
Methods of enhanced oil recovery involving injection of carbon dioxide at high pressures into a reservoir via an injection well while withdrawing reservoir fluids via a production well are well known in the art. Such a process is described by Whorton et al. in U.S. Pat. No. 2,623,596. later it was discovered that recovery of petroleum could be increased in such processes if the carbon dioxide was used in slug form and driven through the reservoir by an aqueous drive fluid such as brine, plain or carbonated water. A typical process of this type is described by Holm in U.S. Pat. No. 3,065,790. Next, it was discovered that the areal sweep efficiency and the overall efficiency of the carbon dioxide-waterflood enhanced oil recovery process could be improved by incorporating a foaming agent and water into the slug of carbn dioxide to form a foam. U.S. Pat. No. 3,342,256 to Bernard et al. describes such a process utilizing a wide variety of foaming agents including water-soluble salts of esters of C.sub.3 -C.sub.6 dicarboxylic acids, such as monosodium dioctyl sulfosuccinate and the like.
In U.S. patent application Ser. No. 621,685, filed Oct. 14, 1975 by Fischer et al., now U.S. Pat. No 4,036,764, there is described a method of drilling and workover in a high temperature well employing as the drilling or workover fluid a foam containing a gas, such as air, carbon dioxide or the like and an aqueous solution of a foaming agent having the formula: ##STR2## wherein R is a straight chain alkyl radical having from 10 to 16 carbon atoms, and M is an alkali metal or ammonium cation, with sodium being preferred. A particularly preferred foaming agent is sodium lauryl sulfoacetate.
It has been the experience that in spite of the wide variety of foaming agents known for foaming aqueous base media, the foaming of a mixture of carbon dioxide and water for use as a sweep fluid in an enhanced oil recovery process presents many problems. The mixture of carbon dioxide and water to be foamed constitutes an acidic composition. Many foaming agents are not stable in acidic media and tend to decompose. Due to the length of time required to carry out an enhanced oil recovery process, it is necessary that the foamed carbon dioxide remain in the foamed state for a prolonged period of time while passing through a reservoir which often has an elevated temperature. Many foaming agents satisfactorily foam carbon dioxide initially, but the foam tends to break down after a brief period of time, such as a day or two. Other foaming agents are capable of foaming carbon dioxide at atmospheric or relatively low pressure but fail to form satisfactory foams under the high pressure conditions at which enhanced oil recovery processes are often carried out. At low pressures, carbon dioxide tends to behave as a gas. At high pressures, carbon dioxide becomes a viscous fluid and takes on many of the characteristics of a liquid. Some foaming agents must be present in a relatively high concentration to foam carbon dioxide. Due to the large volume of injected fluids required in the instant process, it is desired to keep the concentration of foaming agent in the foam to a minimum. Thus, there is a need for an enhanced oil recovery process which employs a carbon dioxide foam which is stable at the conditions under which the process is carried out.
Accordingly, a principal object of this invention is to provide a method of enhanced oil recovery using an aqueous base carbon dioxide foam as a sweep fluid.
A further object of this invention is to provide such a method conducted in a high temperature reservoir.
A still further object of this invention is to provide such a method utilizing a foaming agent which foams carbon dioxide at high pressure.
Another object of this invention is to provide such a method utilizing a carbon dioxide foam which is stable over a long period of time at elevated pressure.
Other objects, advantages and features of this invention will become apparent to those skilled in the art from the following discussion and appended claims.